1. Field of the Invention
The present invention concerns a hearing device of the type having a first microphone that delivers a first microphone signal, a second microphone that delivers a second microphone signal, and a signal processing device to generate a hearing device output signal from the first microphone signal and the second microphone signal. The present invention also concerns a corresponding method for generating a hearing device output signal.
2. Description of the Prior Art and Related Subject Matter
The internal (quantum) noise of the microphones in hearing devices limits the dynamic range, in particular when differential directional microphonal algorithms are used. These algorithms have the result that the noise of the microphones is notably boosted at low frequencies. The higher the selected order of the directional microphone, the more serious this effect because (as is known) the transfer function of a directional microphone of the second order with three microphones drops at low frequencies faster than the transfer function of a directional microphone of the first order with two microphones. Given a directional microphone of the second order, the low frequencies consequently must be more emphasized than the low frequencies in a directional microphone of the first order, such that the signal-to-noise ratio at low frequencies is disadvantageous for a directional microphone of the second order.
Heretofore, the directional effect for low frequencies has been limited, as described, for example in “Three microphone instrument is designed to extend benefits of directionality”, The Hearing Journal, October 2002, vol. 55, nr. 10, pages 38 through 45. Below a specific limit frequency, only a directional microphone of the first order is used, and for high frequencies a directional microphone of the second order is used.
The damping of the interfering signals that do not originate from the preferred direction is also less for low frequencies. In order to reduce the noise for the hearing device user, given quiet input levels the directional effect is therefore completely removed, such that the noise caused by one directional microphone ceases to exist. In this case, the microphone characteristic corresponds to an omnidirectional microphone.
A directional microphone system for hearing devices is known from the German Patent 103 16 287. Such a microphone system can be based on silicon micro-mechanisms. An electret capacitor microphone could alternatively be used.